Metal galvanised coatings consisting essentially of zinc and 0.1 to 0.4% by weight of aluminium are traditionally used for their good protection against corrosion.
These metal coatings are now challenged especially by coatings comprising zinc, and magnesium and aluminium additions of respectively up to 10% and up to 20% by weight.
Such metal coatings are collectively referred to herein as aluminium-zinc-magnesium coatings or ZnAlMg.
The addition of magnesium significantly increases the corrosion resistance against red rust of these coatings, which enables a reduction in their thickness or an increase of the guarantee of protection against corrosion over time at constant thickness.
These sheets are intended, for example, for use in the automotive, electrical appliance or construction fields.
They can be added to paints before or after their finishing by users in these fields. When they are painted before finishing, they are called “pre-lacquered” sheets, wherein the latter are particularly intended for the electrical appliance or construction fields.
In the case of pre-lacquered sheets, the entire sheet metal fabrication method is implemented by the steelmaker, thus reducing the costs and constraints associated with the painting process at the user.
However, it is noted that known metal coatings may be prone to delamination problems of the paint layers, leading to local corrosion of the sheet.